The present invention relates generally to an electrostatographic printing apparatus and more particularly to a fusing system for fixing toner material to support substrate. In particular the present invention relates to a release agent donor member for a toner fixing station in such apparatus.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of an electrostatic latent image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles commonly referred to in the art as toner. The residual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper with subsequent affixing of the image thereto.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent into the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll thereby to affect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with an adhesive material, such as a silicone rubber or other low surface energy elastomer or, for example, tetrafluoroethylene resin sold by E. I DuPont De Nemours under the trademark Teflon. The silicone rubbers which have been used as the surface of the fuser member can be classified into three groups according to the vulcanization method and temperature, i.e., room temperature vulcanization silicone rubber referred hereinafter referred to as RTV silicone rubber, liquid silicone rubber to as LSR rubber, and high temperature vulcanization type silicone rubber referred to as HTV rubber. All these silicone rubbers or elastomers are well known in the art and are commercially available.
In these fusing systems, however, since the toner image is tackified by heat it frequently happens that a part of the image carried on the supporting substrate will be retained by the heated fuser roller and not penetrate into the substrate surface. This tackified material will stick to the surface of the fusing roller and come in contact with the subsequent sheet of supporting substrate bearing a toner image to be fused. A tackified image which has been partially removed from the first sheet, may transfer to the second sheet in non-image portions of the second sheet. In addition, a portion of the tackified image of the second sheet may also adhere to the heated fuser roller. In this way and with the fusing of subsequent sheets of substrates bearing the toner images, the fuser roller may be thoroughly contaminated. In addition, since the fuser roller continues to rotate when there is no substrate bearing a toner image to be fused there between, toner may be transferred from the fuser roll to the pressure roll. These conditions are referred to in the copying art as "offset". Attempts have been made to control the heat transfer to the toner and thereby control the offset. However, even with the adhesive surfaces provided by the silicone elastomers, this has not been entirely successful.
It has also been proposed to provide toner release agents such as silicone oil, in particular, polydimethyl silicone oil, which is applied on the fuser roll to a thickness of the order of about 1 micron to act as a toner release material. These materials possess a relatively low surface energy and have been found to be materials that are suitable for use in the heated fuser roll environment. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner image carried on the support material. Thus, a low surface energy, easily parted layer is presented to the toners that pass through the fuser nip and thereby prevents toner from offsetting to the fuser roll surface.
Some recent developments in fuser members, release agents and fusing systems are described in U.S. Pat. No. 4,264,181 to Lentz et al., U.S. Pat. No. 4,257,699 to Lentz and U.S. Pat. No. 4,272,179 to Seanor, all commonly assigned to the assignee of the present application. These patents describe fuser members and methods of fusing thermoplastic resin toner images to a substrate wherein a polymeric release agent having functional groups is applied to the surface of the fuser member. The fuser member comprises a base member having an elastomeric surface with a metal containing filler therein which has been cured with a nucleophilic addition curing agent. Exemplary of such fuser member is an aluminum base member with a poly(vinylidenefluoride-hexafluoropropylene) copolymer cured with bisphenol curing agent having lead oxide filler dispersed therein and utilizing a mercapto functional polyorganosiloxane oil as a release agent. In those fusing processes, the polymeric release agents have functional groups (also designated as chemically reactive functional groups) which interact with the metal containing filler dispersed in the elastomer or resinous material of the fuser member surface to form a thermally stable film which releases thermoplastic resin toner and which prevents the thermoplastic resin toner from contacting the elastomer material itself. The metal oxide, metal salt, metal alloy or other suitable metal compound filler dispersed in the elastomer or resin upon the fuser member surface interacts with the functional groups of the polymeric release agent. Preferably, the metal containing filler materials do not cause degradation of or have any adverse effect upon the polymeric release agent having functional groups. Because of this reaction between the elastomer having a metal containing filler and the polymeric release agent having functional groups, excellent release and the production of high quality copies are obtained even at high rates of speed of electrostatographic reproducing machines.
While the mechanism involved is not completely understood, it has been observed that when certain polymeric fluids having functional groups are applied to the surface of a fusing member having an elastomer surface with a metal oxide, metal salt, metal, metal alloy or other suitable metal compounds dispersed therein there is an interaction (a chemical reaction, coordination complex, hydrogen bonding or other mechanism) between the metal of the filler in the elastomer and the polymeric fluid having functional groups so that the polymeric release agent having functional groups in the form of a liquid or fluid provides an excellent surface for release which having an excellent propensity to remain upon the surface of the fuser member. Regardless of the mechanism, there appears to be the formation of a film upon the elastomer surface which differs from the composition of the elastomer and the composition of the polymeric release agent having functional groups. This film, however, has a greater affinity for the elastomer containing a metal compound than the toner and thereby provides an excellent release coating upon the elastomer surface. The release coating has a cohesive force which is less than the adhesive forces between heated toner and the substrate to which it is applied and the cohesive forces of the toner. The interaction between the functional group of the polymeric release agent and the metal of the elastomer containing metal leads to an overall diminution of the critical or high surface energy of the metal in the metal containing filler.
U.S. Pat. Nos. 4,029,827, to Imperial et al., 4,101,686 to Strella et al. 4,185,140 also to Strella et al also disclose the use of polymeric release agents having functional groups which interact with the fuser member to form a thermally stable renewable self cleaning layer having superior release properties for electroscopic thermo plastic resin toners, and are all commonly assigned to the assignee of the present invention.
In particular, U.S. Pat. No. 4,029,827 is directed to the use of polyorganosiloxane having mercapto functionality as release agents. U.S. Pat. Nos. 4,101,686 and 4,185,140 are directed to polymeric release agents having functional groups such as carboxy, hydroxy, epoxy, amino, isocyanate, thioether and mercapto groups as release fluids.